shorter film-flange distance opens up a new world of lens design. You don't need retrofocus any more. The RF 50/1.2 or RF 28-70/2.0 simply aren't possible with SLR-style 44mm film-flange.
Also, while Canon's not going there yet, mirrorless can be more compact for given quality. I could get my EOS M+22/2 in my jeans pocket. Granted, a small-sensor, but you couldn't put small-sensor SLRs in your pocket. A 35/2 without IS or macro would be a pancake lens for the R. For travel photography size matters: the SLR shot looks like a cell phone picture, because it is a cell phone picture, because you left the SLR in the hotel...
Finally, while I take your point that you could make an SLR out of any mirrorless body by adding the mirror box, it's always going to be more expensive to do so.
So, yes, other than size, cost, and lens designs, SLRs can do everything that MILFFs can. But those are three pretty important concerns to most people.